Exhaust emissions from a hydrocarbon-fueled engine, such as in an automotive vehicle, are treated in a catalytic converter which has exhaust gas contacting surfaces. The gas contacting surfaces are defined by longitudinal exhaust flow passages (cells) in a metal or ceramic support member such as an extruded ceramic (cordierite) or corrugated metal monolith. A washcoat layer of refractory oxide carrying catalytic metal is retained on the surfaces of the passages. Very fine catalytic metal is dispersed on fine (micron size) particles of refractory oxide, such as alumina. As exhaust gases pass through passages of the monolith, the catalytic metal carried by the washcoat promotes oxidation of unburned or partially oxidized hydrocarbons and carbon monoxide and promotes the reduction of nitrogen oxides. Noble metals such as platinum and palladium are used in treatment of auto exhaust due to their durability at high temperatures. In the case of diesel engines, it is necessary to treat diesel particulates which are typically on the order of 0.1 micron in diameter and contain a solid, carbonaceous portion and a soluble organic portion. Diesel particulate can be collected and removed from an exhaust gas stream by various types of filters or traps. This approach is technically difficult and costly.
It is desirable to treat diesel particulate by continuous catalytic combustion of particulate in the exhaust gas stream over a catalyst in a catalytic converter, in a method similar to the treatment of gaseous hydrocarbon emissions. Converters using support members coated with alumina have been evaluated for this purpose. However, such alumina washcoats promote an undesirable reaction with sulfur which is present in a relatively large amount in diesel fuels as compared to gasoline. Combinations of oxides which include alumina, such as alumina-titania-vanadia mixtures, are also not favored.
Silicas have been used in washcoats but have not been favored for use with vehicle engines operated at high temperatures due to the relatively low thermal stability of silica and the undesired reaction of sulfur with platinum supported on silica. Silica supports doped with metal oxides including TiO.sub.2 and V.sub.2 O.sub.5 have been used for reaction of nitric oxide with ammonia but are not used for treatment of diesel exhaust.
Therefore, it is desirable to have a washcoat which provides improved adherence and reduced reaction with sulfur when used as a support for platinum or palladium.